pixel selection to cut, paste, and move pixels in texture paint
Texture paint as of yet doesn't allow actual selection of pixels in any way close to Photoshop or Gimp do. I have several workarounds that I use though to work from camera view on an image plane and then project from view to new geometry that serves as my duplicated pixels/selection. This is a bit of work for a novice, and only works on 2d images and not when painting on a 3d model with a wrapped texture. Many people just keep the image texture open in a separate image editor program and update the image texture in Blender once saved in PS/Gimp/etc.